Being different from light-emitting display devices such as cathode ray tubes and plasma display devices, liquid crystal display devices are generally provided with a light source (back light) at the back of the liquid crystal panel, in which the light emitted by the light source and transmitted through the panel is utilized for producing images on the panel. As the light source for the back light, generally used is a cold-cathode tube (fluorescent lamp) or the like, to which is applied a high-frequency current as generated in a lighting circuit to have a frequency of from tens to hundreds Hz or so and an effective voltage of hundreds volts or so.
The US UL (Underwriters Laboratories Inc.) Standards, which are the worldwide safety standards for insulated wires for internal wiring in electronic appliances, are applied to the insulating coatings for the insulated wires and shielded wires through which the high-frequency current is supplied to the cold-cathode tube, and to the insulating tubes to be applied over the connections between the cold-cathode tube and the insulated wires, for the purpose of ensuring the mechanical properties of those insulating coatings and insulating tubes and of ensuring the safety thereof against accidental fires. The UL Standards state the details of the initial and heat-aged mechanical properties (tensile strength and elongation), flame resistance, and withstand voltage characteristics of various insulated wires and insulating materials.
At present, the insulating materials for that use in insulated wires and others require flexibility for facilitating wiring work, in addition to the characteristics as stated in the UL Standards. In order to meet this requirement, for example, used are crosslinked compositions that are prepared by making soft polyolefinic resins, such as EVA (ethylene-vinylacetate copolymer) and EEA (ethylene-ethyl acrylate copolymer), resistant to flames followed by exposing the resulting flame-retardant resins to accelerated electron beam, and also silicone rubbers, etc. The degree of flexibility of those insulating materials is about 10 kg/cm.sup.2 or lower in terms of the modulus of tensile elasticity.
For insulated wires comprising insulating materials of those flame-retardant, crosslinked compositions of soft polyolefinic resins, the UL Standards state that the initial tensile strength of the insulating coatings of the insulated wires must be not smaller than 1500 psi (1.05 kg/mm.sup.2) and that the initial elongation thereof must be not smaller than 100%. Regarding the heat-aged physical properties of such insulated wires, the UL Standards state the retention of the initial values, while requiring that the heat-aged tensile strength of insulated wires must be not smaller than 70% of the initial one and that the heat-aged elongation thereof must be not smaller than 65% of the initial one. The heat-aging tests for the physical properties of insulated wires as stated in the UL Standards are, for example, such that samples for a temperature rating of 150.degree. C. are heated in a Geer oven aging tester at 180.degree. C. for 7 days, that samples for a temperature rating of 125.degree. C. are heated in the same at 158.degree. C. for 7 days, and that samples for a temperature rating of 105.degree. C. are heated in the same at 136.degree. C. for 7 days.
As well known, the current demand for compact and lightweight electronic appliances is extremely high. In order to satisfy that demand, it is necessary to provide small-sized internal units.
For example, in electronic appliances comprising liquid crystal display devices, not only the liquid crystal display units but also the peripheral devices of back light units are reduced in size and thinned, and the wiring space for insulated wires for cold-cathode tubes is being narrowed year by year. It is, therefore, important to reduce the diameter of insulated wires.
However, where insulated wires having a reduced diameter are used for high-density wiring in a limited wiring space, the amount of heat generated by the wired circuits increases. In such wired circuits, therefore, the insulating materials to be used require improved heat-aging resistance.
In addition, in such a narrow, limited wiring space, the insulated wires are often kept in contact with the chassis of the back light unit which is at the earth potential. In that case, leakage of high-frequency current from the insulating coatings will increase, often resulting in a great decrease in the supply voltage. As a result, a serious problem will occur that results in lowering the degree of luminance of the cold-cathode tube in the wired device.
For example, the current insulating materials, such as the flame-retardant compositions of soft polyolefinic compositions or silicone rubbers, have a high specific dielectric constant of 3.3 or higher. Therefore, if the insulating coating made of the current insulating material is used for producing insulated wires and if the thickness of the insulating coating is thinned in order to reduce the diameter of the insulated wires, the leak current from the insulated wires increases, often produces the serious problem.
In addition, as a result of the reduction in the wiring space, the insulated wires are often adjacent to the electronic parts in the wired units, in which the noise as derived from the high-frequency current running through the conductor parts will have some negative influences on the electronic circuits, thereby producing another problem of flickering of pictures.
In order to solve this problem, a structure comprising shielded wires may be employed in which the insulating coating formed over the core conductor is further covered with an outer conductor layer such as a metal braided conductor layer or a spirally-covered shielding layer. However, even in those shielded wires, the thickness of the insulating coating must be reduced in order to reduce the diameter of the shielded wires. Therefore, if the conventional insulating material having a high specific dielectric constant is used as the insulating coating in those shielded wires, the capacitance between the core conductor and the outer conductor layer is increased, thereby causing still another problem in that the rising of the wave pattern of the high-frequency current to be transmitted through the shielded wires becomes dull and that the supply voltage is lowered.
On the other hand, silicone rubbers have poor mechanical strength and, if used for producing shielded wires, they are problematic in that the insulating coating of the silicone rubber is cracked after having been rubbed with the metallic material constituting the outer conductor layer, whereby the core conductor and the outer conductor layer are often short-circuited.
For insulating tubes that may be formed over the outer conductor layer in order to insulate and protect the connections between the cold-cathode tube and the insulated wires therearound, generally used are insulating materials similar to those for the insulating coatings for the insulated wires, for example, flame-retardant compositions of soft polyolefinic resins such as those mentioned hereinabove. In those insulating tubes, therefore, it is also an important theme to lower the specific dielectric constant of those insulating materials.